Some of the best investors in the world find success through what others may view as a risky strategy: high portfolio concentration levels.

Despite the Oracle of Omaha overseeing stakes in 43 stocks at Berkshire Hathaway, more than half of his portfolio can be traced to three brand-name businesses.

The stock looks very cheap after this recent fall. The largest investor in Ally is Berkshire Hathaway, the conglomerate run by legendary investor Warren Buffett. The company has held its shares in Ally even though it has trimmed the majority of its exposure to the banking sector in the past year or two.